Polymers are useful in a variety of applications, including fundamental research and biomedical applications such as solid phase diagnostics, drug delivery, biomaterials, and other areas. Unfortunately, many polymer materials are incompatible with a variety of reagents in many applications. For example, proteins and other biomolecules tend to adhere to hydrophobic surfaces, and therefore hydrophobic polymers may not be suitable in some applications where proteins and other biomolecules are involved. Or some type of surface modification/coating will be needed to reduce the non-specific binding (NSB) of these biomolecules to hydrophobic polymer surfaces.
A recent success of fabricating microfluidic devices using an elastic polymer has further increased the utility of polymers in fundamental research and biological applications. Successful fabrication of microfluidic devices from elastic polymers has been particularly useful in small scale sample preparation, chemical synthesis, drug delivery, biomedical devices, and other applications where a micro-scale device is required. Because of a much higher surface area to volume ratio in microfluidic environment, the adsorption of reagents on the surface of the microfluidic devices result in significant changes in reagent concentration and carryover contamination, etc. Therefore, reducing NSB of molecules onto the microfluidic device surface is particularly important.
Recently, surface modification or coating has become a more and more popular way to create “new” materials by selecting core polymer materials for their mechanical properties and then modifying or coating the polymer surface to fit a particular environment. The desired surface properties can be biocompatibility, chemical resistance, wettability, non-stick, etc. Preferably, a surface modification process should be simple, form a covalent bond, and inexpensive. Moreover, it should be mild so as not to degrade the substrate polymer. Commonly used surface modification/coating techniques include plasma deposition, physical vapor deposition, chemical vapor deposition, ion bombardment, ion-beam sputter deposition, ion-beam-assisted deposition, sputtering, thermal spraying, and dipping. Each method has its advantages and disadvantages. For example, if the coating material is not permanently bonded to the polymer, it needs to be reapplied frequently to maintain inertness of the polymer to the reagent.
Inability to produce a permanent coating is particuarly true for polymers with low surface energy, such as, polyethylene, poly(dimethylsiloxane) (PDMS or silicone polymer), poly(tetrafluoroethylene) (PTFE, Teflon®). In addition, microfluidic devices have extremely small features, e.g., flow channels having width of as small as 1 μm. Therefore, coating a surface modifying compound may destroy the microfluidic design or render the microfluidic device unusable.
Conventional permanent bonding (grafting) of a surface modifying compound to an organic or inorganic substrate (i.e., glass, silicon, metal, polymer, etc.) requires activation of the substrate surface, i.e., introducing a reactive functional group on the substrate surface. Typically, the substrate surface is activated by ion deposition. Other methods of surface activation include treating with an oxidizing agent, a reducing agent, or a plasma, such as oxygen plasma or water plasma. After a reactive functional group is introduced on the substrate surface, it is reacted with a surface modifying compound to form a covalent bond. Alternatively, the activated surface is reacted with a linker compound which serves as a linker between the substrate surface and the surface modifying compound. Unfortunately, many linkers and solvents used in these processes are not compatible with a large amount of polymeric materials. Also, surface activation processes increase the cost and time for producing such polymeric devices. Moreover, many polymer surface activation processes result in activation of only a small portion of the polymer surface.
Therefore, there is a need for a process of producing a surface modified polymer that does not require a separate polymer surface activation process. There is also a need for microfluidic devices comprising a covalently attached surface modifying compound.